Protection systems are known to protect secured areas. For example, one known protection system is a fire/security system. Such systems typically rely upon a number of sensors within the secured area that detect some known hazard.
For example, a fire alarm system may include a number of fire sensors (and audio and visual fire warning devices) distributed throughout the secured area or concentrated in the areas of greatest risk. One known fire alarm system, called an analogue addressable fire system, includes one or more 2-wire loops that connect each of the sensors to a central alarm panel.
Analogue addressable fire systems combine power transmission and data communication on a pair of conductors, called a 2-wire loop, between the central panel (i.e., a control and indicating equipment (CIE) panel) and a number of outstations or field devices (e.g., audible alarms, flashers, etc.), which can often exceed 100 devices. In large or even medium sized installations, the length of each loop may exceed 1 kilometer (Km). In some cases, the CIE may be in control of a number of such loops with each loop exceeding 1 Km.
In order to improve reliability, each 2-wire loop may be driven from both ends. In order to further improve reliability, each device connected to the loop may have an in-built short circuit isolator that opens the loop adjacent the device in order to isolate the short circuit.
Included within each short circuit isolator are two voltage detectors, a control element and an isolation device, which may be a relay or MOSFET switch. If a short-circuit occurs, it will pull down the loop voltage below a threshold level. This will cause the isolator to open-circuit one conductor of the 2-wire loop. It should be noted that the isolator and the associated connection to the loop cable could have a combined series resistance of up to 0.1 ohms.
Due to the combined series resistances of the devices on the loop, the loop cable resistance and the current drawn by each of the devices connected to the loop, there is typically a large drop in loop voltage during alarm conditions. This problem can be aggravated where a short circuit or open circuit occurs adjacent to the CIE panel and where the loop can only be driven by one end of the cable.
However, it is often difficult to anticipate difficulties associated with device reliability in this situation. Accordingly, a need exists for better methods of powering devices on 2-wire loops.